Public cellular networks (public land mobile networks) are commonly employed to provide voice and data communications to a plurality of subscribers. For example, analog cellular radiotelephone systems, such as designated AMPS, ETACS, NMT-450, and NMT-900, have been deployed successfully throughout the world. More recently, digital cellular radiotelephone systems such as that designated as IS-54B (and its successor IS-136) in North America and the pan-European GSM system have been introduced. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993. In addition, satellite based radio communication systems are also being utilized to provide wireless communications in various regions such as the Asian Cellular Satellite System (ACeS) generated by Lockheed Martin Corporation.
FIG. 1 illustrates a conventional terrestrial wireless communication system 20 that may implement one of the aforementioned wireless communication standards. The wireless system may include one or more wireless mobile terminals 22 that communicate within a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular radiotelephone network may comprise hundreds of cells, and may include more than one MTSO 28 and may serve thousands of wireless mobile terminals 22.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between wireless mobile terminals 22 and a MTSO 28, by way of the base stations 26 servicing the cells 24. Each cell 24 will have allocated to it one or more control channels and one or channels. The control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the communication system 20, a duplex radio communication link 30 may be effected between two wireless mobile terminals 22 or between a wireless mobile terminal 22 and a landline telephone user 32 via a public switched telephone network (PSTN) 34. The function of the base station 26 is commonly to handle the radio communications within the cell 24 to and from the wireless mobile terminal 22. In this capacity, the base station 26 functions chiefly as a relay station for data and voice signals.
FIG. 2 illustrates a conventional celestial (satellite) wireless communication system 40. The celestial wireless communication system 40 may be employed to perform similar functions to those performed by the conventional terrestrial wireless communication system 20 of FIG. 1. In particular, the celestial wireless communication system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and satellite wireless mobile terminals 23. The satellite 42 communicates with the satellite wireless mobile terminals 23 and earth stations 44 via duplex communication links 46. Each earth station 44 may in turn be connected to a PSTN 34, allowing communications between the wireless mobile terminals 23 and conventional landline telephones 32 (FIG. 1).
The celestial wireless communication system 40 may utilize a single antenna beam covering the entire area served by the system, or as shown in FIG. 2, the celestial wireless communication system 40 may be designed such that it produces multiple, partially-overlapping beams 48, each serving a distinct geographical coverage area 50 within the system's service region. A satellite 42 and coverage area 50 serve a function similar to that of a base station 26 and cell 24, respectively, of the terrestrial wireless communication system 20.
Thus, the celestial wireless communication system 40 may be employed to perform similar functions to those performed by conventional terrestrial wireless communication systems. In particular, a celestial radiotelephone communication system 40 has particular application in areas where the population is sparsely distributed over a large geographic area or where rugged topography tends to make conventional landline telephone or terrestrial wireless infrastructure technically or economically impractical.
In both terrestrial and satellite based communication systems, it is known that a mobile terminal typically must first synchronize with a particular base station on the communication system by locating the appropriate carrier for a control channel of a base station which is capable of transmitting to and, preferably, receiving from the mobile terminal. The communication system typically transmits a synchronization burst during some repeating time portion or segment of a multi-frame of a Time Division Multiple Access (TDMA) system or at recurring intervals in an analog system. Because cellular systems typically use a variety of carriers for control channels in various cells or regions of the geographic area covered by the communication system, the mobile terminal generally searches a plurality of candidate carriers before selecting a carrier for synchronization. For example, the ACeS system includes 170 potential TDMA carriers separated 200 kHz from each other transmitting one m-sequence burst every 470 ms for use in synchronization.
It is also known to facilitate the synchronization process by providing a mobile terminal with a list of candidate carriers to consider when attempting to synchronize with a transmitter of a communication system. The mobile terminal typically tests each of the candidate carriers and selects a best candidate. The selection may be based on comparing the power of the signal received from each of the candidate carriers. A coarse synchronization followed by a fine synchronization are then typically performed with the selected carrier. For a TDMA system, this synchronization typically involves both frequency and time synchronization. After fine synchronization, the mobile terminal generally attempts to read data, such as a broadcast control channel transmission, to access the communication system.
While this approach to synchronization is effective, it can be problematic with low link margin communication systems. For example, satellite communication systems typically require a greater time for each candidate carrier to attempt to establish synchronization. This can result in an undesirable delay between a user seeking communication access, for example, by powering up the mobile terminal, and successful synchronization with the communication system. In addition, a further problem with low link margin communication systems such as satellite systems is that the mobile terminal may be in a blocked position, such as in a building, where it will not be possible for the mobile terminal to successfully receive a signal from the communication system. The synchronization system of the mobile terminal may then consume excessive battery power with repeated unsuccessful attempts to connect to the communication system. Accordingly, there is a need for improved synchronization systems and methods for mobile terminals operating in low link margin environments.